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The quest for a sub-two hour marathon takes another step forward on Friday with Adidas unveiling its adizero Sub2 marathon shoe ahead of Saturday’s Tokyo Marathon. The shoe, with a distinctive blue colorway, will be worn by former world record holder Wilson Kipsang in the race.

Adidas has been working on the shoe for the past two years after the concept came to mind after the 2012 London Marathon, which was won by Kipsang in 2:04:44.

“Around the London Marathon in 2012, we started thinking about Sub2 as a concept and Adidas’ role in achieving what was deemed impossible,” Adidas global general manager André Maestrini said. “We began creating a shoe that could enable this, and Wilson is the perfect athlete to test our innovation in a race environment. We’re incredibly excited to see where this can go.” Nike has joined in the race for a sub-two hour marathon with its own respective project titled Breaking2, where the sportswear giant is training the first sub-two-hour marathon runner and developing its own respective footwear for an attempt to break the barrier later this spring.

What’s so special about the shoe?

One of the adjustments to the adizero Sub2 is the debut of Boost Light innovation. With cushioning made of hundreds of tiny foam pellets aimed at reducing impact while in action, Adidas’ Boost technology has caught on within the running and fashion circles. The shoe’s upper is composed of one single layer of ultralight fabric and weight-reduced mesh with internal reinforcements. The mesh has Microfit technology, which is developed to create the best support, comfort and fit for high-speed road racing. A Continental Microweb improves upon Stretchweb, which was Adidas’ technology developed for the Boost, to improve the grip for any kind of race day conditions.

Adidas’ internal research concluded that Boost can improve running economy by 1%. The new Sub2 shoe’s weight has been reduced by 100 grams and could lead to another 1% improvement in running economy. The Adidas adizero Sub2 will be available to the general public later this year.

"The goal is to be 100 grams lighter than the latest Adizero edition," says Matthias Amm, Adidas' global running category director. "The current shoe that Wilson Kipsang is wearing is around 150 grams."

But what does the 1% increase in running economy actually mean during a race? "When you run a long run or a marathon, at one point, you'll notice a man with a hammer comes out of nowhere and hits you in the legs," Amm says. "That's when your muscles get sore. The improvement in running economy would basically be that you're able to run longer and more easy."

How far away are we from a sub-two hour marathon?

The current world record is held by Kenya’s Dennis Kimetto, who ran 2:02:57 at the 2014 Berlin Marathon while wearing Adidas’ adizero Adios Boost 2.0 racing flats. The past four men’s marathon world records were set by runners wearing Adidas running shoes. Adidas also owns seven of the top 10 fastest all-time marathons, including Boston which is a net downhill course not valid for world records. Kipsang was also wearing the Adios Boost 2.0 when he ran 2:03:23 at the 2013 Berlin Marathon for a then-world record.

When asked about what other companies are doing with their respective "sub-two" shoes, Amm says that there are no springs or shox in the adizero Sub2, just the Boost light technology.

"Within Boost we're constantly innovating and with the Adios we had the first big milestone there. Boost Light is the next major milestone in distance running because it allows us to keep the energy return from Boost but in a much more lighter form," Amm says. "That was a key ingredient for the shoe as well as the Continental stretch on the outsole which allows for less slip and more grip. If you accumulate all those little things, it can make the difference between a world record and non-world record or running 1:59:59 or 2:00:01."

British author Ed Caesar is following the Nike project for Wired and recently wrote the book Two Hours: The Quest to Run the Impossible Marathon. He estimates that a sub-two could be possible by 2025 or even 2020. Back in 1991, Michael Joyner, a polymathic anesthesiologist at the Mayo Clinic in Minnesota, estimated that the physiological limit for a man in the marathon was 1:57:58. He later told SI that he was just hoping to see it in his lifetime.

Kipsang has been working with Yannis Pitsiladis, a professor of sports science at the University of Brighton in England, on their independent Sub2 project, which relies on science and medicine to try to get under the two-hour barrier. Kipsang joined the team shortly after finishing second to Ethiopian runner and Olympic champion Kenenisa Bekele at the 2016 Berlin Marathon. Bekele just missed the world record by crossing the tape in 2:03:03. Kipsang was second in a personal best of 2:03:13, which is tied for the sixth-fastest of all-time.

What was the testing process like?

"We took all the shoes from [marathon world record holders] Haile [Gebrselassie], Patrick Makau, Dennis Kimetto and Wilson to look at the wear of the rubber and how they used the shoes. We put those ingredients together. We had them come in for extensive research with the first prototype. We went back and created a finalized prototype before traveling to Kenya with a small team. We gave them the chance to run in the shoes in fast conditions and with high speed filming. A lot of testing happened with key athletes and local athletes. We're now confident to release the shoe with Wilson," Amm says.